Now here's my theory
Spoiler!
Itachi did not have the legendary items at this point in time. And with sage mode genjutsu won't work on jiraiya very well, as either Ma and Pa frog can disrupt the chakra flow canceling genjutsu when they are fused when ever needed or vice versa. Not to mention Jiraiya has great chakra control a he is a sage mode user and has knowledge and skill in disrupting his own chakra flow evident when Jiraiya was trying teach naruto this. Some say only a sharingan can break Itachi's genjutsu but why would Jiraiya bother teaching this to Naruto if had it no chance of despelling Itachi's genjutsu. Mind you Jiraiya taugh this to naruto with Itachi in mind, Jiraiya acknowldged Itachi as one of their enemies he even told Naruto this very thing.
Now lets review this situation Itachi would be dealing with, has no legendary weapons and his genjutsu, his strength could be negated where Itachi would be wasting considerable chakra. Also lets not forget Itachi's stamina was affected by his sickness in part 2, was his sickness a factor also? I for one can see why Itachi would have such a reaction to meeting Jiraiya at that point in time given these potenial problems. Also Itachi would know more about Jiraiya than Jiraiya about Itachi given the age gap and Jiraiya's reputation as a sannin and a sage in the shinobi world. So Itachi is in a postion to make an accuate judgement call, Itachi is smart this wasnt just to give his fanboys a reason to overhype.
But what happens to sage mode users while under genjutsu? Would it work the same way for genjutsu users when using genjutsu?
You are static when under a genjutsu thats a good thing for sage mode users but what about the fundmantal chakra differences. Genjutsu tries to regulate chakra(spiritual + Physical energies) of the victim but sage chakra has spiritual, physical and natural energies. Would this cause problems? Sage chakra is noted to be a stronger this may make it hard to keep Jiraiya in a genjutsu.
One thing about Jiraiya's way of dealing with genjutsu that gets overlooked. The method of disrutping your chakra flow is to cancel genjutsu when your already under the effects of genjutsu, that's why how fast the genjutsu activates doesnt affect the Jiraiya abilty to break it. But genjutsu users try to cancel genjutsu by trying to detect it by noticing flaws in the in the illusion created(thats why genjutsu users are generally intelligent) so ofcourse guys like Orocihmaru and Kahashi will beaten because Itachi is a better genjutsu user. Particlualy MS genjutsu it activates in seconds.
Jiraiya taught this to Naruto for this very reason because you have be a good genjutsu user to detect it and cancel it before it activates. Thats why Jiraiya told him this method as it involves having chakra control something Naruto can improve on as oppose to suddenly becoming a better genjutsu user than Itachi, the same is for Jiraiya. Jiraiya adimitted hes not a use genjutsu type shinobi and told Naruto he wasnt either.
So the only question is do you think Jiraiya can sustain his mental state long enough to disrupt his chakra while under itachi's MS genjutsu? That is the ONLY argurment against Jiraiya's ability to break Itachi's MS genjutsu. I say he is due his unique training he has undertaken for most of his life being sage mode. Sage training needs a strong spirit(aka guts, will of fire...etc) to undertake the training and Jiraiya out of sage mode demonstrated his strong will, that memoral scene of Jiraiya willing himself back from the dead. And MS genjutsu tries to break ones spirit as mentioned by Kisame here(
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Some are even willing to say Itachi's MS genjutsu can't be broken via chakra disuption because Itachi made a claim to Kahashi in part 1 that only a blood relative can be break his MS genjutsu. If Itachi's MS genjutsu can only be broken via blood relation wouldn't the same be for sasuke's MS jutsu they are both blood related? But his MS genjutsu was broken via chaka discuption by Killer bee. What about during the sasuke vs Itachi fight, did sasuke break Itachi's MS jutsu as soon as he activated his curse seal transformation like we saw in the illusion? Or was it because he was blood related? Or was Itachi just playing around as Itachi as it was later mentioned to he could have killed Sasuke if he wanted? So which is it? Either way the claim of Itachi's MS can only be broken via blood relation has no ground anymore.
There are different reasons why itachi said this its regarding Sasuke to keep him in the village as he just put him in his MS genjutsu shortly after that point in part 1 resulting in a coma. Itachi wanted to Sasuke to day in Konoha in that condition so wouldnt we able to leave but Tsande got rid of Itachi's MS genjutsu on sasuke in part 1 which resulted in sasuke following Orocihmaru something Itachi didnt want to happen. The point to Itachi even coming back to Konoha was for sasuke's sake to begin with but choose to go after Naruto to get the nine tails, this why I love it when Itachi fans still say Itachi spared Jiraiya for Konoha' sakes dispite his plans were only to protect Sasuke at this point. The more you look into this the more you realise nothing is unbeatable in accordance to Itachi's words himself "No jutsu is unbeatable".
As a final note the inability of orochimaru and kahashi of dealing with itachi's genjutsu has nothing to do with jiraiya. He is a completely different shinobi when it comes to dealing with genjutsu. Don't underestimate Jiraiya because of the other genjutsu users( orochimaru and kahashi) failing against Itach without considering his strenghts, Jiraiya is different than the others at least acknowledge this.
Now the rest of the Itachi statement,"even if they were more of us the outcome would not change".